Thermal analysis is a technique monitoring variations in temperature change of certain molten substances during solidification to be able to determine the microstructure and hence properties of the substances in solid form. This is accomplished by taking a sample from the melt, transferring it into a sample vessel and recording and evaluating a time-dependent temperature change in the sample during solidification, by means of temperature responsive means, such as thermocouples or other devices known in the art.
When using thermal analysis for controlling solidification processes in molten materials, such as cast-iron or aluminium alloys, a most critical issue is to bring the sample vessel and the sample quantity as close to thermal equilibrium as possible, and to provide for a controlled, even and reproducible rate of heat removal from the sample. The reason for this is to make it possible to measure temperature changes during phase transformations, the knowledge of which is essential in order to control certain solidification processes.
WO 86/01755 discloses a method for producing compacted graphite cast iron by using thermal analysis. A sample is taken from a bath of molten cast iron, and this sample is permitted to solidify during 0.5 to 10 minutes. The temperature is recorded simultaneously by two temperature responsive means, one of which is arranged in the centre of the sample and the other in the immediate vicinity of the vessel wall. So-called cooling curves representing temperature of the iron sample as a function of time are recorded for each of the two temperature responsive means. According to this document it is then possible to determine the necessary amount of structure-modifying agents that must be added to the melt in order to obtain the desired microstructure.
One example of a sampling device is disclosed in WO 96/23206. The device comprises a container intended to contain a sample quantity of liquid metal during analysis, and a sensor for thermal analysis. The container comprises an inner wall with an interior surface intended to face the sample quantity during analysis, and an outer wall with an exterior surface intended to face the ambient atmosphere. The inner and outer walls are joined at the mouth of the container such that an essentially closed space is formed between the walls.
Another example of a sampling device is disclosed in EP 1 034 419. The container of the sampling device comprises a substantially semi-spherical bottom part having a flattened part. The distance between the walls in the flattened part is less than the distance between the walls in the cylindrical part of the container. Thereby, the sampling device simulates a spherical solidification of the molten metal inside the container, which is the most reliable and accurate shape for thermal analysis, but is not spherical in shape.
Thermal analysis is a heat balance. The ultimate shape, and thus the resolution, of the cooling curve is determined by the balance between the heat liberated during solidification and the heat lost to the sampling device and the atmosphere. It is evident that the amount of heat liberated by the solidification of a 200 gram sample of for example cast iron is fixed. If the 200 gram sample is contained in a vessel that cools quickly, the heat liberated by the solidification will be less able to prevail over the heat loss than it would be in a vessel that cools more slowly. The result is that the faster cooling of the vessel will provide less resolution in the cooling curves. Fast cooling caused by the vessel can also alter the true solidification behaviour of the iron by inducing chill or by influencing the undercooling. In order to extract as much information as possible from the heat liberated by the solidification, it is necessary to design a thermal analysis sampling device such that it neither masks nor dilutes the information provided by the solidification. The other major requirement of a thermal analysis sampling device is that it must ensure consistent sampling conditions. Because the differences in the liberated heat between a good microstructure and an out-of-spec microstructure can be very small, it is critical that all variations measured are due to differences in the iron and not due to differences in the sampling technique.
Even though the above mentioned sampling devices work very well for thermal analysis, they are sometimes difficult to produce because the distance between the walls of the container has to be sufficiently regulated in order to ensure the proper heat transfer during thermal analysis such that the sample quantity of the melt in the sampling device solidifies in the intended manner. If the distance between the inner and outer walls of the container is not carefully controlled, the heat transfer, and thus the solidification of the sample quantity, will be affected such that a reliable measurement is jeopardised. Thus, there is still room for an improved sampling device which overcomes or at least reduces the above mentioned problems.